A photographic material comprises a silver halide emulsion provided on a support. The silver halide emulsion is usually chemically sensitized with various chemical sensitizers to obtain a desired sensitivity or gradation. Examples of the chemical sensitizers include a chalcogen (sulfur, selenium or tellurium) sensitizer, a noble metal (such as gold) sensitizer, a reduction sensitizer and a combination thereof.
An improvement of the silver halide photographic material has increasingly been demanded for several years. The recent photographic material requires very high sensitivity. Further, the photographic image requires an improvement of the graininess and the sharpness. A rapid image forming process such as a quick development process is also necessary. The sensitization and the sensitizers have been improved to meet these requirements.
Sulfur sensitization has most frequently been used in silver halide photography. Numerous sulfur sensitizers have been known and used in silver halide photographic materials.
On the other hand, selenium sensitization is more effective than the sulfur sensitization. Accordingly, selenium sensitizers have been proposed in place of the sulfur sensitizers. For example, U.S. Pat. No. 3,297,447 discloses selenium sensitization and selenium sensitizers. However, the proposed selenium sensitizers have a tendency that fogging easily occurs in the image and gradation of the image is softened. Therefore, the selenium sensitizers should be further improved to be used in place of the conventional sulfur sensitizers.
The above-mentioned chalcogen sensitization is usually used in combination with a gold sensitization. The combinations of the sensitizations remarkably improve the sensitivity of the photographic material. However, the combinations also increase the degree of fog in the image. The fog in gold-selenium sensitization is more remarkable than that in gold-sulfur sensitization. Accordingly, the fog caused by a selenium sensitizer should be reduced, particularly in the case that the selenium sensitizer is used in combination with a gold sensitizer.
Japanese Patent Provisional Publication No. 4(1992)-271341 (having no foreign equivalent) discloses an improved chalcogen sensitizer, which is represented by the following formula. ##STR2## in which each of R.sub.1 and R.sub.2 is an aliphatic group, an aromatic group, a heterocyclic group; and X is Se or Te.
Japanese Patent Provisional Publication No. 5(1993)-11385 (having no foreign equivalent) discloses another improved selenium sensitizer, which is represented by the following formula. ##STR3## in which R.sub.1 an aliphatic group, an aromatic group, a heterocyclic group, --OR.sub.3 or --NR.sub.4 R.sub.5 ; R.sub.2 is --OR.sub.3 or --NR.sub.4 R.sub.5 ; each of R.sub.3, R.sub.4 and R.sub.5 is hydrogen, an aliphatic group or an aromatic group.
Silver halide photographic materials containing the above-mentioned improved sensitizers have a relatively high sensitivity. Further, the materials are almost free from fog. However, the improvement is still insufficient. The silver halide photographic material now requires a further improved chalcogen sensitizer.